Method of making titanium dioxide rectifiers



Filed Jan. 4, 1952 R. G. BRECKENRIDGE METHOD OF MAKING TITANIUM DIOXIDE RECTIFIERS June 12, 1956 INVENTOR.

VOLTS United States Patent lVIETI-IOD OF MAKING TITANIUM DIOXIDE RECTIFKERS Robert G. Breckenridge, Bethesda, Md.,.assignor to the United States of America as represented by the Secretary of Commerce Application January 4, 1952, Serial No. 265,027

3 Claims. (Cl. 29-253) (Granted under Title 35, U. S. Code (1952), see. 266) The invention described herein may be manufactured and used by or for the Government of the United States for governmental purposes without the payment to me of any royalty thereon in accordance with the provisions of the act of March 3, 1883, as amended (45 Stat. 467; 35 U. S. 0.45).

This invention relates to metal or plate rectifiers for electric current and has particular reference to the production of such a rectifier employing a layer of titanium metal in contact with a layer of titanium dioxide containing a small amount of excess titanium as an essential impurity.

Metal or plate rectifiers, as known, usually comprise a stack of elements each of which consists of a metal base plate, a superposed layer of a semiconducting material and a further layer of metal as a counter-electrode, the last layer being usually of a different material from the base plate. Graphite is sometimes used as the counterelectrode. The semiconductors chiefly employed in presently known rectifiers are selenium, cuprous oxide, and magnesium-cupric sulphide. Recently germanium rectifiers have come into widespread use, although perhaps these latter are more correctly classed among the crystal detectors. While the known rectifiers have found considerable use and have attained great commercial importance, they are nevertheless subject to a number of very serious limitations which restrict their utility. Some of these rectifiers, for example cuprous oxide and selenium are restricted in the temperature range at which they can be operated. Heating above the range of 50 degrees C. and 90 degrees C. respectively, will seriously impair or destroy their rectifying action. This imposes a serious limitation in the design of compact apparatus employing such rectifiers and also limits the power which they can handle and dissipate. Other types, for example, selenium rectifiers, require substantial amounts of expensive material. The most important commercial types require extreme care in their manufacture and often do not give consistent results, being very sensitive to changes in manufacturing techniques, so much so that it is very difiicult to achieve completely uniform output. The actual manufacture of these items tends to be an empirical art rather than a science. There is no laboratory test by means of which the raw material can be selected for the manufacture of these rectifiers. At present the only possible method of trial is by the actual production of a rectifier. The presence of even the most minute quantities of undesirable impurities may have an adverse eifect on the reverse resistance of the rectifier as may also the absence of certain essential impurities. The temperature control during manufacture of copper oxide rectifiers, for example, is exceedingly critical, while often closely controlled annealing and quenching techniques are necessary to produce satisfactory results.

The present invention obviates most, if not all, of the above difiiculties and permits the production of a rectifier having very satisfactory electrical characteristics.

It has been appreciated for some time that titanium dioxide has at least theoretical possibilities as a rectifier invention.

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ful rectifiers. The mechanicalproperties of this arrangement are very poor.

According to the present invention commercially pure titanium metal is provided with aqsuitable semiconducting layer'in one step by heating'to a temperature of about 600 degrees C. in an atmosphere of'steam.

Reference will now-be made to-the appended drawings in which Figure 1 is a cross sectionthrough a single element of a rectifier madein accordance with the present Figure 2 is a graph showing the currentvoltage characteristic of such a rectifier.

Referringto Figure l, 1 is aplateor disk-oftitanium which may be supported by and maintained in good electrical contact with a base plate 2 of any other suitable material. The titanium metal is treated as above described to produce a nonporous adherent film of reduced titanium dioxide 3, which is a semiconducting material. The titanium metal 1 is used as the base electrode, electrical contact being made either directly or through the base plate 2. The counter-electrode 4 is made of any suitable metal which may be applied to the titanium dioxide layer 3 by any known process, such as evaporation in a vacuum, metal spraying, electrodeposition, et cetera. The unit so prepared shows a rectifying action; that is, the measured resistance of the unit depends on the direction of current flow and it is nonlinear as shown by the graph of Figure 2.

It will be seen that according to this invention an electrically satisfactory rectifier is produced, having the advantages previously indicated. It is prepared from nontoxic materials of high melting point, which in addition to the advantages previously stated also give a high burn-out resistance. The oxide layer containing some trivalent titanium as an essential impurity is preferably made by heating the titanium in an atmosphere of pure steam (H2O gas) at 600 degrees centigrade maintained for approximately 3 hours. The temperature may be varied from 550 degrees C. to 750 degrees C., and the time from 1 /2 to 4 hours. The thickness of the semi-conducting layer varies linearly with time when the temperature is held constant. Although at the lower temperatures considerably greater time is required this procedure is considered best, since at the upper range of temperatures the oxide layer tends to have poor physical properties. It has been found that the preferred procedure produces a layer in the order of 10 microns thick, which appears to give the best result.

The reaction between titanium metal and steam produces simultaneously by this single step both the quadrivalent oxide and trivalent titanium-ion impurity. It has been found that the resultant film produced in this manner has much better physical properties than films produced in any other way. Uniformity of the product so produced can readily be maintained very closely. There are no particular empirical skills or trial runs required so that this product lends itself very readily to commercial production. The use of steam produces very little embrittlement of the titanium metal, such as tends to be produced by other reduction techniques.

The complete oxide layer is not equally semiconducting and therefore it is desirable to have the counter electrode make good contact with the portions of the oxide layer that give the best rectifying action. In order to accomplish this the semiconductor is placed in a plating bath as the cathode and subjected to an alternating current for about 30 seconds. The better rectifying areas will become plated during this short period while the poorly rectifying areas will not. Then when the counterelectrode is applied to the oxide layer it will make its best contact with the areas of the oxide that are the best semiconductors.

It will be apparent that the embodiments shown are only exemplary and that various modifications in construction and arrangement can be made within the scope of my invention as defined in the appended claims.

I claim:

1. The method of producing a titanium dioxide rectifier comprising the steps of applying a surface of titanium on a base plate to act as the base electrode, heating the titanium in an atmosphere of live steam to form a layer of titanium dioxide, which contains trivalent titanium ions, on the titanium base electrode, and applying a counterelectrode to the oxide layer, said heating taking place within a range of times and temperatures from 1 /2 hours at 750 degrees C. to 4 hours at 550 degrees C.

2. The method of producing a dry disk rectifier comprising the steps of applying a layer of titanium on a base plate to form a base electrode, subjecting the titanium to an atmosphere of live steam at 600 degrees C. for 3 hours to form a semiconducting layer on the titanium and applying a counterelectrode to the semiconducting layer.

3. The method of producing a dry disk rectifier, comprising the steps of applying a layer of titanium on a base plate to form a base electrode, subjecting the titanium to an atmosphere of live steam at 600 degrees C. for 3 hours to form a semiconducting layer of titanium dioxide on the titanium, placing the base electrode and semiconductor in a plating bath so as to form the cathode thereof, passing alernating current through the bath for 30 seconds, and applying a counterelectrode to the oxide layer.

References Cited in the file of this patent UNITED STATES PATENTS 294,559 Wood Mar. 4, 1884 1,985,118 Van Geel Dec. 18, 1934 2,139,791 Barrey Dec. 13, 1938 2,379,919 Miller et al. July 10, 1945 FOREIGN PATENTS 648,423 Great Britain Ian. 3, 1951 

1. THE METHOD OF PRODUCING A TITANIUM DIOXIDE RECTIFIER COMPRISING THE STEPS OF APPLYING A SURFACE OF TITANIUM ON A BASE PLATE TO ACT AS THE BASE ELECTRODE, HEATING THE TITANIUM IN AN ATMOSPHERE OF LIVE STREAM TO FORM A LAYER OF TITANIUM DIOXIDE, WHICH CONTAINS TRIVALENT TITANIUM IONS, ON THE TITANIUM BASE ELECTRODE, AND APPLYING A COUNTERELECTRODE TO THE OXIDE LAYER, SAID HEATING TAKING PLACE WITHIN A RANGE OF TIMES AND TEMPERATURES FROM 1 1/2 HOURS AT 750 DEGREES C. TO 4 HOURS AT 550 DEGREES C. 